The present invention relates to axle pivoting controllers and hydraulic cylinders for industrial vehicles.
A forklift is generally provided with a rear axle beam and a body frame. In order to improve the riding comfort and enhance the driving performance of the forklift, the center of the rear axle beam can be supported such that the rear axle beam is pivotal with respect to the body frame. The body frame and the rear axle beam are connected to each other by a hydraulic cylinder.
Japanese Unexamined Utility Model Publication No. 56-25609 describes a rear axle beam having ends that are each connected to a body frame by a single action hydraulic cylinder. Each hydraulic cylinder has a piston and an oil chamber. The oil chambers of the hydraulic cylinders are connected with each other by a passage. In accordance with the pivotal movement of the rear axle beam relative to the body frame, each piston is moved axially in its associated cylinder. Hydraulic oil flows between the oil chambers through the passage in accordance with the movement of each piston and restricts the pivotal movement of the rear axle beam.
When such a forklift carries a cargo, lifts a cargo to a high position, or changes directions at a high speed, the forklift becomes less stable. In order to increase the driving stability, the pivotal movement of the rear axle beam can be locked by restricting the movement of the pistons. An electromagnetic control valve is arranged in the passage to restrict the movement of the pistons by stopping the flow of hydraulic oil in the passage.
A forklift that restricts pivotal movements of the rear axle beam by employing two single action hydraulic cylinders requires a large number of components, which are installed on the forklift. In order to reduce the number of components, the owner of the present application has proposed to arrange a multiple action hydraulic cylinder on just one end of the rear axle beam to connect the rear axle beam with the body frame. The multiple action hydraulic cylinder has a piston, which defines a first oil chamber and a second oil chamber. The first and second oil chambers are connected with each other by a passage. The piston is moved axially in accordance with the pivotal movements of the rear axle beam. This moves the hydraulic oil between the first and second oil chambers and restrains the pivotal movement of the rear axle beam relative to the body frame. In this structure, the electromagnetic control valve restricts the movement of the piston by stopping the flow of the hydraulic oil in the passage between the first and second oil chambers. Thus, the pivotal movements of the rear axle beam are also restricted with this structure. In addition, this type of forklift employs only one cylinder. Therefore, the installation of the hydraulic cylinder is facilitated due to the smaller number of components.
However, in the multiple action hydraulic cylinder, one end of the piston is connected to a rod, which extends through one of the hydraulic oil chambers. Thus, the cross sectional area of this chamber, on which pressure is applied, is smaller than that of the other hydraulic oil chamber. Accordingly, the pressure applied to the piston differs when the piston moves in opposite directions. As a result, the velocity of the piston differs according to the moving direction. The difference in the velocity of the piston permits the forklift to tilt to the right and to the left in different manners such that the operator can feel the difference. In addition, the volume of oil that flows outward from one oil chamber is not the same as the volume of oil that enters the other oil chamber. It is thus necessary to eliminate the imbalance between the two oil volumes and permit the forklift to tilt to the right and to the left in the same manner.